Things to Know




#Genetic Drift




#Gene Flow




#Directional Selection

Review: is a population evolving?




  • Re-examine the beetle populations
    • color: one locus, 2 alleles


  • Across 2 generations, are allele frequencies the same?


  • Is this population in Hardy Weinberg equilibrium?

Hardy Weinberg describes a population NOT evolving

Allele frequency change in beetles: Mutation



  • Mutations in the beetle color gene, resulting in more brown
    • to rare to change allele frequencies on their own
    • require synergy with a another driver (e.g., selection)


  • Mutation rate = probablity that one allele changes to another via mutation
    • mutation rates are generally small (~0.0001%).
    • humans = 2.5 x 10-8 mutations nucleotide-1 generation-1
    • negligible change from one generation to the next


  • Mutation ARE the ultimate source of genetic diversity

Allele frequencies change in beetles: Genetic Drift


  • Genetic drift describes how allele frequencies may fluctuate unpredictably across generations
    • happens in every population
    • pronounced in small populations
    • extremely important evolutionary “force”


  • Reduces genetic variation through the random loss of alleles


  • In a small population, beetles are green or brown
    • several green beetles get stepped on by Dr. C
    • next generation has more brown beetles, by chance
    • did it matter if alleles were dominant or recessive? or better suited? or helped with mating?

Hypothetical coin flips to choose 2 alleles

  • Expected frequencies of alleles are just mathematical probability (not certainty)


  • In small populations should we expect the allele frequencies to remain identical across generations?

Mechanism of Genetic Drift: Founder Effect




  • Founder Effect: when a few individuals become isolated from a larger population and establish a new population
    • some alleles are kept, others are lost
    • allele frequencies likely change by chance


  • Occurs frequently on islands
    • what could happen with bad alleles?

Mechanism of Genetic Drift: Bottleneck Effect



  • Bottlenecks occurs when an event drastically reduces a population size, likely altering allele frequencies
    • sudden change in the environment
    • fire, flood, invasive species, etc.


  • The new gene pool may be different from original population


  • If the population stays small, genetic drift could increase across populations
    • black robins on Chatham island

Bottlenecks, species habitat loss, and conservation

Allele frequencies can change through Gene Flow


  • Gene flow: exchange of alleles between 2 or more populations
    • aka ‘Gene migration’
    • like mutations but way more frequent


  • Occurs when individuals migrate and mate successfully
    • brown beetles enters population
    • changes allele frequency for color


  • Gene flow can be good or bad
    • may increase genetic diversity
    • may bring harmful alleles


  • Does gene flow tend to make populations more or less unique?

Allele frequencies change in beetles: Natural Selection

Only natural selection causes adaptive evolution!


  • ‘Sorting’ of alleles that favor reproductive success
    • leads to adaptation at population level


  • Certain traits lead to greater Relative Fitness
    • individuals with beneficial traits (alleles) will contribute more to gene pool
    • better survival/reproduction compared to others


  • More individuals in a population with greater relative fitness means….


  • Natural selection is a ‘moving target’…WHY?

Natural Selection: selection of heritable traits

Natural Selection: Directional selection of heritable traits

Natural Selection: Diversifying selection of heritable traits

Natural Selection: Stabilizing selection of heritable traits